The present invention relates to a thread-cutting insert for cutting screw threads into a workpiece.
Thread cutting inserts have been proposed which comprise a plurality of cutting teeth along one edge thereof. The teeth are of progressively increasing heights, as exemplified for example in U.S. Pat. No(s). 4,572,713 and 4,854,785.
When a metal tube material that has been subjected to rolling or drawing operations is to be provided with a threaded portion, a thread cutting insert is used to cut a thread into the tube. The insert, when engaging the outer surface portion of the tubular material, imposes a large amount of stress into a surface portion with small radii of profile. Further, the material is often non-circular which means that the insert is subjected to intermittent stress especially when threads on a conical profile are to be cut.
It is furthermore known that an insert having a large nose radius, or a circular insert, has an extended lifetime when engaging with a rolled surface area at intermittent stress than compared with an insert having a small nose radius.
When forming threads by successively cutting multiple groove profiles into the material by means of multiple cutting teeth, it is desirable to cut the completed thread with as few engagements or passes as possible. This means that it is desirable to cut with as large a feed depth as possible especially when cutting the first profile.
With prior art inserts for multiple thread cutting, the profiles of the teeth have been basically similar, but have a progressively varying dimension. This gives the result of engaging the material with a first tooth having a profile with small radii which are sensitive for deformation. This is disadvantageous especially when cutting threads in non-circular profile material where intermittent stresses are obtained.